1. Field of the Invention
This invention relates to a line concentrator used in a data communication system or the like, and more particularly to an ac type line concentrator provided with a crosspoint switch circuit capable of switching an analog signal in which a received carrier signal is transferred through the crosspoint circuit from one of a plurality of incoming lines to a selected one of a plurality of outgoing lines without being demodulated.
In a data communication system which is provided with a line concentrator, a central unit and a plurality of terminal units, m lines on the terminal unit side (hereinafter referred to as incoming lines) are concentrated into n (n &lt; m) lines on the central unit side (hereinafter referred to as outgoing lines), thereby to increase the communication efficiency of the transmission lines and thus attain the economical use thereof.
2. Description of the Prior Art
Various line concentrators have been proposed hitherto. FIG. 1 shows, by way of example, a conventional line concentrator for use in a data communication system.
Referring to FIG. 1, the data communication system is provided with a plurality of terminal units 1, a plurality of modulating and demodulating means 2, 3, 7 and 8 (hereinafter referred to as a modem), a line concentrator 4, n outgoing lines 5, m (m &gt; n) incoming lines 6, and a central unit 9. In the data communication system having such an arrangement, a data signal supplied from one of the terminal units 1 is modulated by the associated modem 2, and the output of the modem 2 is transferred to the associated modem 3 in the form of a carrier frequency signal. The modem 3 demodulates the received carrier frequency signal to transmit the data signal to the line concentrator 4, and simultaneously supplies to the line concentrator 4 a carrier detection signal (hereinafter referred to as a CD signal). Upon reception of the CD signal indicating the reception, the line concentrator 4 connects a specific incoming line 6 associated with the detected CD signal to an idle and available one of the outgoing lines 5. The data signal supplied to the selected one of the outgoing lines 5 through the specific incoming line 6 connected thereto is modulated by the associated modem 7 again, and the output of the modem 7 is transferred to the associated modem 8 which demodulates the modulated signal again to transmit the data signal to the central unit 9. When the central unit 9 receives the data signal transmitted from a specific terminal unit 1, the central unit 9 can also transmit a data signal to the specific terminal unit 1.
However, in such data communication system, the modem 3 and the modem 7 respectively associated with each incoming line 6 and each outgoing line 5 have been inevitably required since the method of data communication comprises the steps of demodulating a carrier frequency signal into dc signal form before applying the same to the line concentrator 4, establishing the connection between the incoming and outgoing lines 6 and 5 through the crosspoint switches in the line concentrator 4, and then modulating the dc signal again by the modems 7 and 3 for the transmission of data between the central unit 9 and the specific terminal unit 1. Thus, the data communication system shown in FIG. 1 has been defective in that it is complex in structure and therefore uneconomical. Further, this data communication system has been quite expensive due to the fact that the number of these modems 3 and 7 is necessarily increased with the increase in the number of the incoming and outgoing lines, and the floor area required for the installation of these modems is correspondingly increased.